In particular, the term “fibrous material” refers to fibres of filtering material (for example cellulose acetate) intended to form a continuous filter rod in a single or double-line machine, for producing smoking articles such as filters of cigarettes, both alternatively to the fibres of tobacco intended for forming a tobacco rod in a single or double-line machine, to produce smoking articles such as cigarettes The rod-shaped elongated element can thus be a continuous filter rod, or a filter rod section or a piece of filter in a smoking article, or a continuous tobacco rod, or a piece of tobacco rod in a cigarette.
In particular the present invention relates to the inspection of a continuous rod containing fibrous material, such as fibres of tobacco or fibres of filtering material, to which the following discussion will refer explicitly without thereby any loss of generality.
In order to make a continuous rod of fibrous material, in the tobacco industry, supplying a continuous web of material is known, for example paper material, to a forming beam that wraps the continuous web around a fibrous material deposited thereupon.
In particular, the continuous web is supplied to a forming apparatus, in particular the continuous web is supplied along a path in an advancement direction in contact with a belt conveyor to a forming beam, through a loading station at which the belt receives the fibrous material deposited placed centrally thereupon. The forming beam comprises folding means, i.e. a variable-section groove, in which the belt conveyor and the continuous web engage to be deformed transversely, thus progressively wrapping the fibrous material and making a rod-shaped tubular wrapper.
A ribbon of adhesive material is arranged parallel to an end border of the continuous web on an inner surface of the latter to be brought into contact with an outer face of the tubular wrapper at the forming beam and, when stabilised by pressing means, make the continuous rod.
The continuous rod is advanced at the high operating speeds of current machines of the tobacco industry and is, as part of usual quality control procedures, inspected during advancement thereof before being supplied to a cutting device for cutting the continuous rod into pieces.
One known inspection method determines a diameter of the continuous rod at an inspection station, so that they can be optionally rejected, after cutting of the continuous rod into pieces, the pieces having a diameter that is different from that of a pre-set ideal diameter.
Typically, this inspection method is performed by an inspection group comprising pneumatic apparatuses that obtain the diameter of the rod, processing a difference between an air jet emitted transversely onto the rod and the pressure of the jet received after traversing of the rod. This pneumatic inspection is usually carried out in adjoining inspection portions along the entire extent of the tobacco rod.
In order to improve the aforesaid inspection method, an alternative inspection method has been proposed, which enables not only the diameter of the continuous rod to be measured but also possible defects in the shape thereof, such as for example the ovalness thereof. This alternative method is of optical type and is based on the acquisition, from several directions, of a plurality of shadows of the continuous rod that enable the shape of the continuous rod to be reconstructed in an inspection portion.
In order to detect the shape of the continuous rod, an inspection group is used that comprises a plurality of inspection stations arranged along the longitudinal axis of the continuous rod, in each of which there is an optical assembly formed by an optical image acquisition apparatus and by an illuminator.
The expression “optical image acquisition apparatus” means an optoelectronic image acquisition apparatus, which is able to acquire images of an object, and in particular to process the images so as to extract features of interest from the images, for example geometric and/or shape features, of the object. The acquired images can be in colour or in black and white and from such images, information on the colour (tone, saturation etc.) or respectively on the level of grey and on the light intensity can be extracted.
The optical apparatus usually comprises a body on which an electronic sensor is arranged, for example an alignment or array of photosensitive elements of linear or two-dimensional matrix type, for example of the CCD or CMOS type, and appropriate optical receiving means that is fixed to the body, for example an objective consisting of one or more lenses, by means of which the sensor is suitable for receiving the light diffused from the object to be acquired. The number of pixels or dot-shaped elements that make up the representation of a raster or bitmap image in the memory of the optical apparatus corresponds to the number of photosensitive elements of the electronic sensor. It should be noted that an image with resolution of (n*m) pixel can be obtained by single acquisition using a two-dimensional, or matrix, sensor of (n*m) photosensitive elements.
A control device is comprised in an optical apparatus for controlling the acquisition of the image, the switch on of the illuminator and also, in some applications, for processing the acquired image in order to extract features of interest from the image and communicate the result of the optical inspection to an external control system. Optical apparatuses of this type are known as linear or matrix cameras, and if they are capable of processing the acquired image for analysing information of interest, they are also called “smart cameras”.
The result of the optical inspection performed by the inspection group is communicated to an external control system, for example the control system of the cigarette packaging machine suitable for treating the pieces of continuous rod obtained after cutting, via a high speed data transmission communication network of the Ethernet type or other type. Alternative communication means can also be provided, that is made by a set of digital input or output signals from the optical apparatus, connected to respective analogous digital output and input signals of the control system of the packaging machine.
The control system of the cigarette packaging machine is thus able to reject directly (or to communicate a defectiveness message to an outer device that performs the rejection operation) the inspected piece judged not to conform to the required quality requirements as soon as the piece reaches a rejection station.
In order to detect the shape of the continuous rod, each optical assembly is arranged at a different tilt with respect to the continuous rod, to illuminate from several directions the continuous rod and acquire from different directions partial shadow views projected therefrom. The different partial views acquired by the different optical apparatuses are processed to reconstruct a closed section profile of the continuous rod, the shape of which is analysed in order to extract the diameter and possible faults thereof, for example ovalness of the continuous rod. The inspection portion of the continuous rod thus extends for a segment comprised between the two partial views at the ends.
The need for a plurality of optical assemblies has some drawbacks.
Owing to the plurality of necessary apparatuses, the cost of the inspection group is very high. Further, the inspection group has considerable overall dimensions because the optical apparatuses and the respective illuminators occupy a lot of space along the continuous rod.
Each inspection portion is thus extensive and this implies that at the high advancement speed of the rod, the shape of the continuous rod can be reconstructed with partial views that acquire segments of continuous rod that are also distant from one another along the longitudinal axis of the rod, which can cause great inspection imprecision.
In order to reduce the space occupied by the inspection group, it has been proposed to use a single optical apparatus and a single illuminator that are fixed to a support, the support rotating with respect to the continuous rod, to acquire in succession during the rotation of the support the different shadows projected from different directions in respective partial views. The time necessary for a single inspection is high and also in this case, at the high advancement speeds of the rod, the shape of the continuous rod is reconstructed with partial views that are also very distant from one another.
It follows that an inspection method using a single rotating optical assembly may also be very imprecise, using partial views that are distant from one another in the rod to reconstruct the shape of the continuous rod.
US 2006/109485 discloses a method and an inspection group for inspecting rod-shaped articles of the tobacco industry by a plurality of stripes of light. A plurality of projectors of stripes of light are arranged around the article to be inspected and for each stripe of light a curved height profile is analysed. The curved height profile is acquired by a PSD sensor, i.e. a sensor that is sensitive to position. Precision in reconstructing the rod-shaped article based on the shape of the curved profile thus depends on the respective arrangement of the projector and PSD sensor and thus on the distance between the PSD sensor and the elongated element.
If this arrangement differs from what is configured in a configuration step of the system, for example because of a size change of the inspected article that provides different positioning of one or more projectors of the stripes of light, it is necessary to configure the system again through the intervention of an operator.
Nevertheless, if over time because of vibrations or a positioning error the arrangement between the projector and PSD sensor is no longer the ideal one, there is no longer any certainty that the acquired curved profile of heights corresponds to a real acquisition of the section profile of the rod-shaped article and erroneous acquisitions of the profile can occur.